Vanguard 2 (or Vanguard 2E before launch) is an Earth-orbiting satellite launched 17 February 1959 at 15:55:02 GMT, aboard a Vanguard SLV-4 rocket as part of the United States Navy's Project Vanguard. The satellite was designed to measure cloud cover distribution over the daylight portion of its orbit, for a period of 19 days, and to provide information on the density of the atmosphere for the lifetime of its orbit (about 300 years). As the first weather satellite and one of the first orbital space missions, the launch of Vanguard 2 was an important milestone in the Space Race between the United States and the Soviet Union. Vanguard 2 remains in orbit.

Project Vanguard was a program managed by the United States Navy Naval Research Laboratory (NRL), which intended to launch the first artificial satellite into low Earth orbit using a Vanguard rocket. as the launch vehicle from Cape Canaveral Missile Annex, Florida.

In response to the launch of Sputnik 1 on 4 October 1957, the U.S. restarted the Explorer program, which had been proposed earlier by the Army Ballistic Missile Agency (ABMA). Privately, however, the Central Intelligence Agency (CIA) and President Dwight D. Eisenhower were aware of progress being made by the Soviets on Sputnik from secret spy plane imagery. Together with the Jet Propulsion Laboratory (JPL), ABMA built Explorer 1 and launched it on 1 February 1958 (UTC). Before work was completed, however, the Soviet Union launched a second satellite, Sputnik 2, on 3 November 1957. Meanwhile, the spectacular televised failure of Vanguard TV3 on 6 December 1957, deepened American dismay over the country's position in the Space Race.

On 17 March 1958, Vanguard 1 became the second artificial satellite successfully placed in a low Earth orbit by the United States. It was the first solar-powered satellite. Just 15.2 cm (6.0 in) in diameter and weighing 1.4 kg (3.1 lb), Vanguard 1 was described by then-Soviet Premier Nikita Khrushchev as, "The grapefruit satellite". Vanguard 1, and the upper stage of its launch vehicle, are the oldest artificial satellites still in space, as Vanguard's predecessors, Sputnik 1, Sputnik 2, and Explorer 1, have decayed from orbit.